1. Field of the Invention
The present invention relates to addition-crosslinking silicone compositions which comprise an adhesion promoter, to silicone elastomers prepared from the silicone compositions (S), to a process for preparing the addition-crosslinking silicone compositions, to composite materials comprising the addition-crosslinking silicone elastomers, and to a process for producing composite materials comprising the addition-crosslinking silicone elastomers.
2. Description of the Related Art
Composite articles composed of different materials are becoming increasingly more important as engineering materials. A principle requirement for these materials is a bond between the individual substrate materials that is strong and is durable under the corresponding service conditions.
Possible substrate materials include for example metals, glasses, ceramics, organic polymers, or biological materials. Silicones and crosslinkable silicone compositions, in particular, are playing an increasing part as substrate materials. On account of their adhesive nature, however, these silicones and silicone compositions make it difficult to achieve a durable bond.
Numerous technologies for producing a strong and durable join between silicones and other substrate materials are known.
Fundamentally there exists the possibility of modifying the chemical and physical nature of the substrate material in order to improve the strength of adhesion between the silicone and the substrate material.
One exemplary process is the pretreatment of the surface of the substrate materials by UV irradiation, flame treatment, corona treatment, or plasma treatment. In the course of such pretreatment steps, the surface or near-surface layer of the substrate materials becomes activated—that is, functional groups, primarily polar groups, are created that allow a joint to come about, and that in this way contribute to the realization of a durable composite material.
Another way to produce durably strong silicone composite materials is the application of priming systems, known as primers, to the substrate material. Such primers, however, often contain solvents as well as adhesion-promoting additives, and these solvents must be removed again following application to the substrate material.
A disadvantage of all of these techniques is that the pretreatment of the substrates necessitates an additional operating step.
This disadvantage can be obviated by providing suitable functional groups in the volume of the substrate material, and/or at the surface of the silicone and/or substrate material, which contribute to the development of adhesion in the production of the composite article.
EP0601882 describes for example, a composite material comprising a silicone elastomer and an organic plastic, wherein a polycarbonate substrate material which comprises additional aliphatically unsaturated groups is used.
EP0143994, for its part, describes providing an organohydrogenpoly-siloxane-containing organic plastic which, after vulcanization with an addition-crosslinking silicone composition, allows the production of a durably strong composite material.
However, the procedure just described necessitates a structural modification to at least one of the substrates, and this may adversely affect the physical and chemical properties. Moreover, chemical modification of polymers in some cases is associated with a considerable cost and complexity.
Another way of achieving adhesion between a silicone and a polymeric substrate is to add specific additives and/or specific crosslinkers, known as adhesion promoters. These additives, admixed to the uncrosslinked silicone compositions, develop adhesion to a substrate material during and/or after vulcanization, sometimes only after storage. As a result, there is no decisive intervention in the chemical nature of the materials participating in the bond. Generally speaking, moreover, there is no need for any further pretreatment of the substrate materials.
EP0875536 describes a self-adhesive silicone rubber composition which comprises at least one alkoxysilane and/or alkoxysiloxane containing an epoxy group, and also a crosslinker having at least 20 SiH groups, with the proviso that the SiH:SiVi ratio is at least 1.5.
An additive which is particularly preferred is glycidyloxypropyltrimethoxysilane (Glymo®), which allows relatively high strengths of adhesion to be obtained in composite materials, especially with certain organic plastics. The crosslinkers with 30 SiH groups per molecule that are described in the examples of EP0875536, however, have the disadvantage that crosslinkers with such high functionality lead to a considerable reduction in shelf life, as a result of viscosity increases (stiffening), and hence that, ultimately, the processing quality of the silicone compositions is adversely affected as well. A critical disadvantage of the use of epoxy-functional alkoxysilanes/siloxanes is the elimination of the alcohol group(s), the use of reactive and polar groups, and, in the case of functional alkoxysilanes, the problem of “efflorescence” and “exudation”. The elimination of the alcohol may on the one hand be detrimental to effective adhesion, since the alcohol accumulates at the surface of the silicone and hence also at the interface with the substrate, thereby impairing contact between silicone surface and substrate surface. Furthermore, it is preferred to use methoxy-silanes, which release methanol, which is classified as toxic. Furthermore, the liberation of volatile cleavage products (alcohol elimination) is accompanied by observation of a not inconsiderable contraction of the silicone elastomer, and this, generally, is undesirable.
EP1106662 describes self-adhesive, addition-crosslinking silicone compositions comprising an organosilicon compound which contains not only an epoxy group but also a hydrolyzable group, and an organohydrogenpolysiloxane which has at least one aromatic C6 ring, leading to very good strength of adhesion on numerous organic plastics and various metals. The organohydrogenpolysiloxane here functions both as an adhesion promoter and as a crosslinker. A disadvantage of the compositions described in EP1106622, however, is that the stated additives, first, bring about accelerated SiH breakdown on account of their relatively high reactivity, and, moreover, the crosslinking rate is reduced (inhibition effect).
EP1510553 discloses self-adhesive, addition-crosslinking silicone rubber compositions comprising (A) an organohydrogenpolysiloxane, (C) an adhesion-promoting compound of the general formula (R12)5Ph-Xr-Ph(R12)5, and, as additional adhesion promoter (D), an organopolysiloxane which carries at least one terminal SiH group. These compositions, however, have the disadvantage that Si—H-terminal polymers, which are frequently also used for chain extension, have an adverse effect on the quality of processing in an injection-molding operation, this being manifested in a narrower processing window.
EP0601883 describes self-adhesive, addition-crosslinking silicone rubber compositions which comprise a silane group or siloxane group as an adhesion promoter, which comprise at least one aromatic group and also at least one SiH function. The self-adhesive, addition-crosslinking silicone rubber compositions described are notable for effective adhesion to the stated organic plastics and for a low strength of adhesion to metals. The storage-stable preparation of the adhesion promoters recited in the examples, however, is considered fundamentally to be very complex and therefore costly, and this is ultimately reflected in a reduced economic efficiency.
EP0686671A2 describes a self-adhesive, addition-crosslinking silicone composition which uses no special adhesion promoters, since the adhesion-promoting constituent either is an organohydrogenpolysiloxane which possesses on average per molecule at least two SiH groups and at least 12 mol % of whose monovalent Si-bonded radicals are composed of hydrocarbon radicals having an aromatic ring, or is a compound of this kind which possesses on average per molecule at least one SiH group and which comprises a group consisting of two aromatic rings, the two aromatic rings being separated from one another by —R13R14Si—, —R13R14SiO—, —OR13R14SiO— or —R13R14SiOR13R14Si—, the radicals R13 and R14 being monovalent hydrocarbon radicals. The adhesion-promoting constituent may thus also be the crosslinker of the silicone elastomer composition. This composition produces good adhesion on organic plastics, especially acrylonitrile-butadiene-styrene (ABS) copolymer, but at the same time exhibits easy demoldability from metals. The high level of radicals comprising aromatic rings in the SiH-containing, adhesion-promoting constituent, of more than 12%, however, results in a considerable incompatibility with the other constituents of the addition-crosslinking silicone elastomer composition. This leads on the one hand to partial separation (exudation) during storage, hence necessitating repeated homogenization of the component comprising this constituent, prior to use. This incompatibility, which is evident even in a milky haze of the uncrosslinked composition, is also manifested in a significantly reduced transparency on the part of the silicone elastomer components produced from the material. Where the adhesion-promoting constituent also functions as a crosslinker of the silicone composition, the incompatibility results in vulcanization defects, which lead to inhomogeneous network formation and deficient mechanical vulcanizate properties. In order to circumvent these vulcanization defects, it is necessary, in addition to the adhesion-promoting SiH-containing constituent, to use an SiH-containing crosslinker which is fully compatible with the silicone composition—but this has the consequence of other disadvantages, such as increased values for compression set and increased exudation tendency on the part of the adhesion-promoting constituent, for example. The high level of radicals comprising aromatic rings in the SiH-containing, adhesion-promoting constituent, of more than 12 mol %, also causes a severe structural viscosity of the silicone elastomer composition, and in many applications this is undesirable, as in the case of the injection molding of liquid silicone rubber, for example.
EP0728825B1 describes self-adhesive silicone rubbers which as crosslinker comprise R3Si(OSi(R)H)nOSiR3, —(OSi(R)H)n— or R4-1 Si(OSi(R)2H)1 with n being at least 3 and 1 being 3 or 4, and a specific adhesion promoter. There is therefore explicitly no description of copolymers which contain H(R)SiO2/2 and (R)2SiO2/2 units. The specified adhesion promoter is a compound which comprises at least one aliphatically unsaturated group and at least two phenylene groups.
DE102007044789 describes self-adhesive, addition-crosslinking silicone compositions which exhibit very good adhesion to engineering plastics, especially plastics based on bisphenol A. The adhesion promoter used in this case is a mixture of cyclic organohydrogensiloxanes and a compound having at least two phenyl units and one alkenyl unit.
According to the prior art, there are a large number of known self-adhesive crosslinkable silicone compositions. A problem, however, is the fact that the majority of the adhesion-enhancing additives which are added to the silicone compositions, comprise phenyl groups or alkoxysilyl groups, and these additives, and/or their hydrolysis products, are therefore toxic or injurious to health. As a result, their use in industrial materials is limited. For medical, cosmetic, or near-food applications, these materials are unsuitable.